Direct Current-to-Direct Current (DC-to-DC) converters are often optimized for high load conditions, with their performance degrading during light load conditions. High performance in terms of high efficiency is required in a wide range of loads, such as watts when the product is active, and microwatts when the product is idle.
A DC-to-DC converter may operate under various operating conditions (e.g., input voltage and load current) or different product variants must be supported. Therefore, different external components (e.g., inductors and capacitors) must be supported. A controller that is digitally implemented is preferable. However, in light load conditions, care must be taken such that the power consumption of the digital controller does not heavily contribute to power losses, severely reducing overall conversion efficiency.
Several control techniques for DC-to-DC converters operating in light load conditions have been proposed. Typically these techniques involve reduced switching frequency when the load drops to a low value. This is an obvious technique considering that losses of DC-to-DC converters are mainly caused by conduction losses (due to finite resistances of components, such as power MOSFETs) and switching losses (due to energy spent in the commutation of switching and charging/discharging of gate capacitances of the power MOSFETs). When the load decreases, the conduction losses also decrease, and switching losses become the dominant contributor. Therefore, the switching frequency should be reduced in this range in order to maintain high efficiency. In light load conditions, the DC-to-DC converter is typically operated in Pulse Frequency Modulation (PFM) and the switching frequency is typically decreasing with decreasing load.
A DC-to-DC controller that optimizes light load efficiency is typically implemented in the analog domain in order to save power. A digital implementation is normally not attractive because of the extra losses caused by the Analog-to-Digital Converter (ADC) and by the digital core itself. However, a digital controller is attractive for other reasons, such as flexibility and programmability.